This invention relates to a scanning optical device comprising at least a light source, a collimator lens into which a light beam emitted from said light source enters, a deflector which deflects the light beam having passed said collimator lens to the direction of main scanning, and an image forming lens which focuses the light beam having passed said deflector on the surface to the scanned and to an image forming apparatus having a plurality of the above-mentioned scanning optical devices arranged in the direction of sub-scanning.
Generally speaking, in an image forming apparatus capable of forming a multi-color image, a cylinder-shaped, or belt-shaped image bearing member is used. In forming a multi-color image by using this image bearing member, charging, exposure and development are carried out for each of colors with the image bearing member rotated (moved), and the multiple color toner images are superposed one after another on the image bearing member and transferred onto a sheet of transfer paper by a one-time transfer operation.
Incidentally, in order to accomplish a high-speed printing, it is required not a structure such that exposure processing (writing by light) for one color is carried out in every rotation of the image bearing member but a structure such that the exposures for all of four colors Y, M, C, and K are carried out in every rotation of the image bearing member. In this case, not only a plurality of the developing units but also a plurality of the scanning optical devices corresponding to the respective colors should be provided in order that the multiple exposures and developments should be done at different positions on the image bearing member.
For example, in the case where a color image is formed using the four colors Y (yellow), M (magenta), C (cyan), and K (black), the scanning optical devices and the developing units for the respective colors Y, M, C, and K are arranged in the direction of sub-scanning, and each of the scanning optical devices is let to form the latent electrostatic image of the color corresponding to it in such a manner as to form the toner images precisely superposed on one another, and the latent electrostatic images are developed.
In this case, to remark one pixel of the latent images formed by the respective scanning optical devices, if the position of this pixel is deviated for a certain color, synthesis of the color can not be done at this pixel portion, producing a color deviation and lowering resolution, and image quality is deteriorated. In order to prevent the occurrence of this color deviation and the lowering of resolution, it is required that not only the characteristics of the respective scanning optical devices are made to be the same so as to be able to write the same scan line, but also the respective optical devices are fixed in a precise positional relationship so as to make the scan lines for the respective colors coincide with one another.
To consider the characteristics of the scanning optical device as an individual one, it is desirable for obtaining a good image that the scan lines on the surface to be scanned are straight and, at the same time, the arrangement of the pixels forming any one of the scan lines has a uniform interval between any adjacent pixels. However, actually, the positioning of optical parts etc can not be precisely performed, therefore, there have been problems that scan lines on the surface to be scanned are curved and that the arrangement of the pixels forming a scan line has an uneven interval between pixels.
Further, in the case where a multi-color image is formed, there has been a problem that the characteristics of the respective scanning optical devices can not be made equal, neither the respective scanning optical devices can be fixed in a precise positional relationship, resulting in deterioration of image quality.